This invention relates to antenna testing apparatus and more particularly to a system and procedure for measuring and tuning circularly polarized antennas to maximize their performance.
Antennas can be used in any system that transmits or receives radio frequency signals and are designed to optimize the radiation or reception of electromagnetic energy in some direction while suppressing it in others. During the antenna design process, various antenna properties are measured to evaluate compliance with design requirements. Three important properties of an antenna are polarization, input impedance, and resonant frequency.
Polarization of a radiated wave is that property of an antenna that describes the shape and orientation of the electric field vector as a function of time. To fully characterize the polarization of an antenna, it is necessary to determine the polarization ellipse, axial ratio, and sense-of-rotation of the electric field vector radiated by the antenna. For circular polarized antennas, axial ratio is an important criterion for the performance characterization of the antenna especially when measured at the zenith of the antenna because it is well understood that the zenith measurement can be used to infer axial ratio at other elevations. Since the axial ratio is defined as the ratio of the major axis to the minor axis of the circular wave polarization ellipse, a perfect circular polarized wave corresponds to an axial ratio equal to one, and deviations from unity deteriorate the performance of the antenna. Additionally, the sense-of-rotation or handedness describing the circle in the plane of polarization provides a complete description of the propagation pattern of the circular polarized wave. There are four primary methods that may be employed to measure polarization. These include the polarization-pattern method, rotating-source method, multiple-amplitude-component method, and phase-amplitude method. Typically, these methods require use of a specialized test facility, such as an anechoic chamber, complex instrumentation for frequency stability, elaborate computer controlled electromechanical systems to rotate the probe or antenna element, and are extremely time consuming, especially if tuning of the antenna element or multiple polarization parameter measurements are required. Furthermore, these methods require translating and resolving multiple coordinate reference systems, which may lead to inaccuracies. Finally, in a production environment, utilizing any of these methods would require substantial infrastructure and personnel investment.
The input impedance of the antenna is another important factor in considering maximum power transfer and useful bandwidth of the antenna at its resonant frequency. It is generally a function of frequency and is affected by the antennas physical geometry, method of excitation, and proximity to other objects. It is measured at a single port of the antenna and should be obtained under normal operating conditions. Furthermore, impedance becomes a significant parameter to control for small high frequency operating antennas. Measurement of input impedance can be performed using impedance bridges or broadband network analyzer systems that automatically display complex reflection and transmission parameters, but is accomplished independently of other antenna tests and often in special configurations and ranges.
The above mentioned systems determine circular polarization axial ratio, sense-of-rotation of the polarized wave, impedance matching, and resonant frequency, but require elaborately equipped test facilities, particular configurations for each test, specialized operator expertise, and are time consuming, especially when antenna adjustments are required. The present invention is an antenna test apparatus and method in which the fundamental figures of merit for antenna performance prediction, zenith axial ratio, sense of polarization rotation, impedance, and resonant frequency, can be determined, and tuned if necessary, simultaneously for an antenna element in a production environment requiring only simple test fixtures, vector automated network analyzer, and computer.
The present invention relates to antenna performance measurement and tuning for circularly polarized antenna elements. In one aspect of the invention, the amplitude and phase of signals received at the test fixture probes from the illuminating wave emitted by the antenna under test (AUT) are processed to determine its zenith axial ratio and sense-of-rotation. In another aspect of the invention, the antenna impedance matching and resonant frequency characteristics are measured. An antenna test apparatus is disclosed which comprises a fiberglass epoxy support structure, radio frequency (RF) echo absorber material, conducting ground plane, and antenna probe, that, in combination with a vector automated network analyzer, present amplitude and phase data that is used to determine zenith axial ratio, sense-of-rotation, impedance matching properties, and resonant frequency of the AUT.
It is an object of the present invention to provide a method and means for simultaneously determining the zenith axial ratio, sense-of-rotation, impedance, and resonant frequency of a circularly polarized antenna, which are fundamental figures of merit for antenna performance predictability.
It is a feature of the present invention to provide a test apparatus that is simple to assemble, reliable, occupies minimal space, uses common and familiar test instrumentation, and requires no time-consuming reconfiguration of the AUT for multiple antenna parameter measurements.
It is an advantage of the present invention to provide a method of analysis that is accomplished quickly, readily understood, not prone to errors, and applicable to a broad family of circularly polarized antennas, which permits optimal antenna performance evaluation in a field or production environment.
These and other objects, features, and advantages are disclosed in the specification, figures, and claims of the present invention.